The water cycle explains the continuous movement of water on, above, and below the surface of the earth. It is also referred to as the Hydrological Cycle. The cycle describes the properties of water that make it undergo the various movements on the planet. The water cycle has nine main physical processes that create a continuous water movement on the planet.
Intricate sequences include the transition of water from the gaseous composition of the atmosphere; through the water bodies such as oceans, lakes, rivers; passage through the soil, rocks and underground waters; and later returns into the atmosphere. Simply put, the hydrological cycle has neither a beginning nor an end, it’s an incessant process.
The water cycle processes involve evaporation, condensation, precipitation, interception, infiltration, percolation, transpiration, runoff, and storage.
Evaporation takes place when water changes from its liquid state to vapor or gaseous state. A substantial heat amount is exchanged during the process, roughly 600 calories of energy per gram of water. In most cases, the solar radiation and additional causes such as the wind, vapor pressure, atmospheric pressure, and air temperature influence the amount of natural evaporation in different geographical regions.
Evaporation occurs over the surfaces of the water bodies such as oceans, streams, and lakes. It can also occur on raindrops, rocks, snow, soil or vegetation. When evaporation happens, anything present in the water such as salts and minerals is left behind. Thus, evaporation purifies the water. The evaporated moisture then rises into the atmosphere from the evaporation sources as water vapor or in a gaseous state. At any particular moment, some water vapor is present in the atmosphere.
Condensation is the process whereby the water vapor changes from its gaseous physical state to liquid or crystal solid. The water vapor condenses on minute air particles due to the cooling of the air, freezing temperatures, or increased vapor amounts to the point of saturation in the upper stratospheres.
The condensed vapor then forms fog, dew or clouds. When the condensed clouds, dew, and fogs become too large and heavy to remain suspended in the atmosphere, they fall back on earth as precipitation due to gravity. The 600 calories of energy per gram of water needed during evaporations are released into the environment.
Precipitation takes place whenever any or all forms of water particles fall from the atmosphere and reach the earth surface. Precipitation occurs when the liquid or solid particles in the clouds, dew, and fog drops to the ground because of frictional drag and gravity.
One falling particle leaves behind a turbulent wake, causing faster and continued drops. The crystallized ice may reach the ground as ice pellets or snow or may melt and change into raindrops before reaching the surface of the earth depending on the atmospheric temperatures.
Precipitation falls on water bodies or on ground surface where it disperses in various ways. For sometime, precipitation can remain on the surface as runoff or overland flow. It may be carried into waterways, intercepted by plants, or infiltrate into the soil. A good percentage of precipitation goes back to the atmosphere as evaporation.
Interception is whereby the water movement is interrupted in the various paths during transportation events over the land surface. Interception takes place when the water is absorbed by vegetation cover and trees, absorbed into the ground, or stored in puddles and land formations such as furrows and streamlets. These waters can either infiltrate into the soil or return to the atmosphere through evapotranspiration or evaporation.
Infiltration is the physical process involving the slow passage of water through the soil. This phenomenon is influenced by the soil surface conditions such as permeability and porosity of the soil profile. Other factors include soil texture, soil moisture content, and soil structure. The infiltrated water is stored in the soil and can later return to the atmosphere via evapotranspiration.
Percolation is the flow of water through the soil and rocks by the influence of capillary and gravity forces. All water on the earth’s surface move by the forces of gravity and capillarity to rest beneath the earth as groundwater. Once beneath the earth, below the water table, the water mostly moves horizontally rather than downwards based on the geologic boundary formations.
This area normally acts as reservoirs for storing water. Some geologic formations may conduct this water back to the surface such as springs.
Transpiration is a process in all plants that normally takes place during the day, giving off water vapor from the leaves openings. Plants transpire to move nutrients to the upper sections of the plant and to cool the plants. Most of the water absorbed by the plants are transpired into the atmosphere until a water deficit point is reached whereby the plant resorts to releasing water vapor at a much slower rate. Transpiration is important in the water cycle because plants absorb the moisture from the soil and releases it into the atmosphere as water vapor.
Runoff is the occurrence of excess water from watershed or drainage basin that flows on the surface. The flow is as a result of precipitation above waterways, groundwater runoff from deep percolations, subsurface runoff that infiltrates the surface soils, and surface runoff that flows on the land surface. As the water flows, it can be used for agricultural and domestic purposes, it may seep into the ground, stored in reservoirs or water bodies, or evaporate into the atmosphere.
Storage refers to the various water reservoirs in the planetary water or hydrological cycle. The water is primarily stored in the atmosphere, the surface of the earth, and in the ground. Storage in the atmosphere is in the form of water vapor. Storage on the surface of the earth includes lakes, oceans, rivers, glaciers, and reservoirs. Storage in the ground pertains to the soils, rock formations, and aquifers.